Method and composition for producing phosphate coatings on metal



nite States Patent 2,839,439 Patented June 17, 1958 METHOD ANDCOMPOSITION FOR PRODUCING PHOSPHATE COATINGS ON METAL John MarvinStapleton, Detroit, Mich., assignor to Detrex Chemical Industries, Inc,a corporation of Michigan No Drawing. Application June 7, 1955 SerialNo. 513,907

4 Claims. (Cl. 148-615) This invention relates to a method andcomposition for producing phosphate coatings on metal. Morespecifically, this invention concerns a method and material forproducing on iron or steel a coating consisting predominately of ironphosphate.

For some years, the ferous metal coating art has distinguished sharplybetween the so-called coating phosphates and the so-called non-coatingphosphates. The coating phosphates comprise those phosphates of metalssuch as zinc and manganese for example, which metals are incorporatedinto the coating as a result of the coating process. On the other hand,coating solutions of alkali metal dihydrogen phosphates are commonlyreferred to as the non-coating phosphates. They form fine grained,strongly adherent iron phosphate coatings on ferrous metal, and thealkali metal component of the phosphate coating chemicals does not enterinto the coating formed on the surface of the metal. Examples of thesenoncoating phosphates are sodium, potassium, ammonium, etc.

As the phosphate coating art developed, many chemical accelerators wereproposed and successfully used, both for the coating phosphates and forthe non-coating phosphates. Such accelerators included nitrates andnitrites, as well as varieties of other substances, most of which actedas oxidizing agents and were gradually consumed in the process. Theaccelerators of the prior art have all had varying degrees ofeffectiveness in conjunction with the non-coating phosphates.

Various methods have been employed to prepare the surface of the metalprior to the deposition of the phosphate coating. Examples aresolventdegreasing, alkali cleaning, emulsion cleaning, sand blasting, etc. Ofthe methods listed above for cleaning and preparing the surface of themetal, one very effective, economical and commercially used process isalkali cleaning, wherein the metal is subjected to the action of strongalkali solution containing alkali metal hydroxide, carbonate, phosphate,silicate, etc., or preferably mixtures thereof. Treating the metal in astrong alkali solution and subsequently rinsing in water beforesubjecting the ferrous metal to the phosphate coating bath aitects thesurface of the metal in such a manner that it is ditficult toconsistently produce a fine grained, uniform ten'aciously adherentcoating of iron phosphate on the metal when utilizing conventionalnon-coating phosphate baths. Also, the alkali carryover from the rinse,into the phosphating solution, increases the pH of the bath whichadversely affects the coating efiiciency.

It is accordingly an object of this invention to overcome thedifiiculties just discussed. Another object is to provide a non-coatingphosphate bath containing an accelerator which is very effective evenwhen present in minute proportions. Still another object of thisinvention is to provide a method and solution for providing a fine,uniform, tenaciously adherent iron phosphate coat- 2 ing on a ferrousmetal. Still another object is to provide a method of providingphosphate coatings on successive ferrous metal objects which carry smallquantities of alkali on their surfaces, and of maintaining the phosphatecoating bath at a proper pH notwithstanding such alkali carryover.

Other objects and advantages of this invention will become apparenthereinafter.

In accordance with this invention ferrous metal ob jects to be coatedare first subjected to an alkali cleaning operation, for example bytreating the metal with a so1ution containing any proportions of thenormal ingredients of an alkali cleaner such as caustic soda, soda ash,sodium phosphate, silicate, etc. Normally these solutions are hot whenused. The cleaning step may be accomplished by immersion, spraying, orany other suitable procedure. The metal objects are then removed fromthe alkali cleaning solution and are rinsed in water.

It has been discovered that excellent, fine grained, uniform,tenaciously adherent iron phosphate coatings may be formed on the alkalitreated metal surface after rinsing with water by treating the metalsurface with a phosphate coating solution consisting essentially ofalkali metal dihydrogen phosphate containing an accelerator whichconsists essentially of alkali metal nitrate combined with a smallproportion of alkali metal molybdate. Preferably, the coating solutionrun-oh is collected for reuse. It has further been discovered that, whenthe coating solution run-off is collected for re-use, it may bemaintained at a substantially constant pH notwithstanding the build-upof alkali from the carryover, by adding phosphoric acid to the solution.

In referring herein to alkali metal dihydrogen phosphates, alkali metalnitrate, and alkali metal molybdate, it is of course intended that thesodium, potassium and ammonium compounds of those radicals: be included,together with lithium and the other well known alkali metals.

Alkali metal molybdates in accordance with this invention include thenormal molybdates (MoO -2), the dimolybdates (Mo O -2), trimolybdatesand other polymolybdates up to decamolybdates (Mo O -2) and above. Forexample, commercial ammonium molybdate is (NH4)6MO7O24.4H2O.

Although the relative proportions of the ingredients of the phosphatecoating solution may vary within wide limits, such proportions in theconcentrate are preferably as follows:

Percent by weight Phosphoric acid 1.00 to 6.0 Alkali metal dihydrogenphosphate 10.00 to 37.5 Alkali metal nitrate 2.00 to 13.5 Alkali metalmolybdate .04 to 1.2

Water to make 100.00.

In the working solution, the preferred proportions are as follows:

Percent by weight An alkali such as sodium carbonate or sodium hydroxideand free phosphoric acid is incorporated into the. coating solution, asrequired to maintain the preferred optimum pH range.

3 Example 1 In accordance with a preferred embodiment of this invention,a concentrated solution was prepared consisting of:

Percent by weight The working solution was prepared by adding the aboveconcentrate to water in the volume ratio of 3 parts concentrate to 97parts water. The above solution will have a pH of between 2.9 and 3.3depending upon the water used. The bath will deposit uniform finegrained, tenaciously adherent iron phosphate coating on ferrous metal.However, it is desirable to adjust the pH of the bath to about 4.2 bythe addition of the proper amount of alkali such as soda ash or causticsoda and thereafter the continuous processing of ferrous metal throughthe bath will result in a gradual change in respect to pH, whereby thebath will operate between a pH of 4.2 and 5.8 and continue to produceuniform, fine grained, tenaciously adherent iron phosphate coatings. Thedesired pH range is 4.6 to 5.2, the recommended optimum pH being 4.8. Ametal was prepared for the coating operation by alkali cleaning,followed by a Water rinse. The working phosphate coating solution wasthen sprayed on the rinsed ferrous metal to produce an iron phosphatecoating, after which the metal was rinsed with water, treated withchromic acid, and painted.

As small quantities of alkaline cleaner are carried from the alkalicleaning stage into the water rinse stage and from the rinse into thephosphate stage, the pH of the phosphate coating solution tends to riseabove optimum. When this pH exceeds 5.2, sufficient phosphoric acid isadded to the working solution to lower the pH to the optimum operatingcondition, preferably pH 4.5. One suitable material for adjusting thispH has the following composition:

Percent Phosphoric acid (75%) 83 Water 17 Successive metal objectscoated in accordance with this example are quickly provided with finegrained uniform iron phosphate coatings of excellent adherence andbonding properties for the subsequent application of paint.

Other specific examples of working solutions in accordancev with thisinvention are as follows:

4 Example 4 Percent by weight Phosphoric acid (75%) 0.15 to 0.20 Monosodium dihydrogen phosphate 0.90 to 1.05 Sodium nitrate 0.45 to 0.75Water 98.4998 to 97.9996 Sodium molybdate 0.0002 to 0.0004

Example 5 Percent by weight Phosphoric acid (75%) 0.16 to 0.24 Monosodium dihydrogen phosphate 0.75 to 1.10 Sodium nitrate 0.50 to 0.70Water 98.5898 to 97.9597 Sodium molybdate 0.0002 to 0.0003

Example 6 Percent by weight Phosphoric acid (75%) 0.100 to 0.200

M o no potassium dihydrogen phosphate m. 0.750 to 0.900 Ammonium nitrate0.300 to 0.750 Water 98.8498 to 98.1496

Potassium molybdate (expressed as K2MQ10031) to 100.0000 100.0000

Example 7 Percent by weight The specific gravity of the aboveconcentrate is 1.320 at To each lbs. of the concentrate, 22.5 grams oftechnical sodium molybdate are added. This concentrate is added to thebath in the required volume to give the required total acid content. Thebath is adjusted to the desired pH of 4.6 by the addition of Na CO orNaOH, and metal articles are coated as heretofore described. The totalacid is between 9 and 11 points. If the pH of the bath increases to5.2-5.8, phosphoric acid is added to reduce the pH to 4.6.

Although specific forms of the invention have been described herein forthe sake of clarity, it will be appreciated that equivalents may besubstituted for the components and method steps specifically described,and that certain features of the invention may be used independently ofother features, all within the spirit and scope of the invention asdefined in the appended claims.

Having thus described my invention, I claim:

1. Ina -method of producing an adherent uniform phosphate coating on aferrous metal, the steps which comprise spraying said metal with anaqueous solution of alkali metal dihydrogen phosphate, and acceleratingthe coating action by incorporating into the solution an acceleratorconsisting of about 0.4 to 0.76% by Weight nitrate and about .0002 to.0005% by Weight molybdatt: ions.

2. An aqueous metal coating solution consisting es sentially of about0.922% to 1.05% by Weight non-coating, alkali metal dihydrogenphosphate, about 0.40% to 0.76% by Weight nitrate ions, and about0.000296 11: 0.0005% by weight molybdate ions.

3. A composition for coating ferrous metals consisting essentially ofPercent by weight Phosphoric acid (75%) 1.00 to 6.0 Alkali metaldihydrogen phosphate 10.00 to 37.5 Alkali metal nitrate 2.00 to 13.5Alkali metal molybdate .04 to 1.2

Water to make 100.00.

4. A composition for coating ferrous metals consisting essentially of:

Percent by Weight

1. IN A METHOD OF PRODUCING AN ADHERENT UNIFORM PHOSPHATE COATING ON AFERROUS METAL, THE STEPS WHICH COMPRISE SPRAYING SAID METAL WITH ANAQUEOUS SOLUTION OF ALKALI METAL DIHYDROGEN PHOSPHATE, AND ACCELERATINGTHE COATING ACTION BY INCORPORATING INTO THE SOLUTION AN ACCELERATORCONSISTING OF ABOUT 0.4 TO 0.76% BY WEIGHT NITRATE AND ABOUT .0002 TO.0005% BY WEIGHT MOLYBDATE IONS.